1. Field of the Invention
The present invention relates to production of ceramic material powder and, more particularly, to a method for producing ceramic material powders useful as a material for ceramic electronic parts.
2. Description of the Prior Art
in general, ceramic electronic parts have been produced by using a ceramic composition prepared by incorporating additional metal elements into a basic ceramic composition to improve electric characteristics and sintering property of the ceramics. Such additional metal elements or additives have been incorporated into a basic composition such as, for example, a dielectric ceramic composition of barium titanate by forming a coating of compounds of the additives on surfaces of individual particles of the basic ceramic composition. The coating of the compounds of the additives is generally formed on individual particle surfaces by the following three methods:
(1) a dry method including the steps of mixing carbonates or oxides of additive metal elements with a powder of a basic composition of a ceramic dielectric, grounding the resultant mixture, and calcining the mixture;
(2) a wet process including the steps of adding an aqueous solution containing ions of additive metal elements to a slurry of a basic composition of a ceramic dielectric, adding a precipitant to the mixture to form precipitates of metal ions, filtering off the resultant mixture, drying and then calcining the same; and
(3) a spray method including the steps of mixing a powder of a basic composition with an organic binder and an aqueous solution of metal compounds of additives to prepare a slurry, spray-drying the resultant slurry to prepare granules, and then calcining the resultant granules.
However, these methods have the following disadvantages awaiting a solution. For the dry method, it is impossible to homogeneously and microscopically disperse carbonates or oxides of the additives between powders of the basic ceramic composition because of large particle sizes of the additives. This causes local non-uniformity in concentration of the additives in the basic ceramic composition, which in turn causes wide variations in electric characteristics of the ceramic electronic parts, thus making it difficult to obtain desired characteristics.
The dispersion of the additives is improved by the wet process at a certain degree, but it is insufficient for the production of ceramic electronic parts with desired characteristics. In addition, it is difficult to precipitate all the additive ions simultaneously by addition of one precipitant. For example, ions of Sr, Ca and Mg can be precipitated as a complex carbonate by reactions with carbonic acid ions (CO.sup.-2), but Ti.sup.4 ions and the like can not be precipitated by reaction with carbonic acid ions. Since alkali metals have bad influences on the electrical properties of the ceramic electronic parts, it is preferred to avoid use of alkali salts as the precipitant. However, if an ammonium salt containing no alkali metal is used as a precipitant, some metal elements such as Zn, Mn, Ni and Co form soluble amine complex salts, thus making it impossible to produce precipitates of compounds of these metal.
The spray method occasionally causes production of gel by interaction between the binder and anions or cations present in the mixture for preparation of the slurry, thus making it impossible to prepare particles of the basic composition uniformly covered with additives to uniformly adhere to particle surfaces of a basic composition.